1. Field of the Invention
The present invention relates to a high intensity discharge lamp ballast apparatus.
2. Description of Related Art
Recently, metal halide lamps have been used as the headlights of an automobile instead of halogen lamps. The metal halide lamp, one of the HID (High Intensity Discharge) lamps, is characterized by its high luminous efficiency, high color temperature and long life compared with the halogen lamp. The arc tube of the metal halide lamp contains metal halides which are mixtures of some metals such as sodium and scandium with halogen such as iodine, high pressure xenon serving as a starting gas, and mercury. The metal halide lamp starts emitting light as follows. First, it starts discharge of the xenon in a gaseous condition at room temperature, followed by arc discharge of the xenon, which increases the temperature inside the arc tube. As the temperature in the tube increases, the mercury vaporizes and starts arc discharge, thereby further increasing the temperature in the tube. A further increase in the temperature in the tube brings about the evaporation of the metal halides, followed by their arc discharge, thereby achieving the high color temperature emission at high luminous efficiency. Incidentally, although the mercury serves as a stopgap of the discharge between the xenon and metal halides, metal halide lamps without containing the mercury have been provided recently.
In the ballast apparatus of the metal halide lamp, the densities of the substances in the gases vary greatly in the various phases of the temperature rise in the lamp ballast process. Thus, it is necessary for the ballast apparatus of the metal halide lamp to prevent the discharge from fading away because of a decrease in the electronic temperature, thereby continuing the discharge. In other words, it must control the discharge in response to the variable load characteristics of the lamp. Consequently, the ballast circuit of the metal halide lamp must meet unique requirements which differ greatly from the requirements for ballast circuits of fluorescent lamps (low-pressure mercury vaporization discharge lamps) widely used as normal household lighting and backlights of liquid crystal displays.
As the ballast apparatus of vehicle headlights using the conventional metal halide lamps, the technique disclosed in Relevant Reference 1 is known, for example. In the present specification, the circuit configuration described in Relevant Reference 1 is called “full-bridge, low-frequency ballast system” because of its circuit characteristics. Although the full-bridge, low-frequency ballast system can meet the requirements necessary for the metal halide lamp and implement a rather compact and inexpensive ballast apparatus, further reduction in size and cost is required.
On the other hand, as for the conventional ballast apparatuses of fluorescent lamps used as the backlights of liquid-crystal displays, a thoroughgoing reduction of their size and cost has been carried out as in a ballast apparatus described in Relevant Reference 2, for example. As a result, the systems have been widely used which convert voltages fed from DC power supplies to AC waves using push-pull DC-AC inverters to light the lamps at high frequencies. In the present specification, the circuit configuration disclosed in the Relevant Reference 2 is called “one-stage high frequency boosting inverter system” from its circuit characteristics. The one-stage high frequency boosting inverter system carries out the power conversion from DC to AC only once within the ballast circuit using a push-pull DC-AC inverter. Thus, it can simplify the circuit configuration, and miniaturize its transformer occupying a large portion of the total volume of the apparatus because of the high frequency turn-on, thereby being able to achieve the reduction in size and cost of the ballast apparatus. To apply the circuit scheme to the ballast apparatus of the metal halide lamp, the unique requirements of the metal halide lamp must be met, which prevents the implementation thereof.
Relevant Reference 1: Japanese patent application laid-open No. 2002-352989.
Relevant Reference 2: Japanese patent application laid-open No. 7-211472/1995.
In addition, the full-bridge, low-frequency ballast system separates its boosting DC-DC converter from the inverter for converting DC into AC to satisfy both the requirements to reduce the size of the transformer and to stabilize the high intensity discharge lamp. Thus, it requires the multi-stage conversion, which increases the number of the circuit components, and prevents the reduction in size and cost of the high intensity discharge lamp ballast apparatus.
It is necessary for the ballast apparatus of the metal halide lamp to generate a high voltage pulse of about 20 kV or more across the lamp to cause breakdown during the discharge start period. The full-bridge, low-frequency ballast system supplies the lamp with the high voltage pulse by applying the output voltage of the DC-DC converter to the transformer for starting electric discharge, which is called “ignitor transformer” from now on. In this case, the generating circuit of the high voltage pulse must increase the voltage to be supplied to the primary side of the start transformer to a considerable level to prevent an increase in the size of the ignitor transformer.
On the other hand, as for the circuit using the one-stage high frequency boosting inverter system, the transformer of the DC-AC inverter must have a large turn ratio or a booster circuit at its secondary side to generate the high voltage pulse at the start of the discharge. This increases the size of the transformer, and the number of the components, thereby presenting a problem of hampering the reduction in size and cost of the high intensity discharge lamp ballast apparatus.
In addition, to relight the metal halide lamp after extinguishing the arc after lighting the lamp for a while, the full-bridge, low-frequency ballast system has a discharge growing capacitor (capacitor for assisting to start electric discharge) with a comparatively high withstand voltage and comparatively large capacitance at the secondary side of the transformer to prevent the discharge from fading away, thereby supplying an enough voltage for maintaining the discharge of the metal halide lamp immediately after the discharge start. This presents a problem of hindering the reduction in cost and size of the high intensity discharge lamp ballast apparatus.
On the other hand, the circuit using the one-stage high frequency boosting inverter system employs the same frequency for driving the transformer and for ballasting the lamp according to its circuit configuration. As a result, to supply the lamp with the voltage necessary for the discharge growth, the transformer must have a large turn ratio or a capacitor with a large capacitance at the DC circuit portion on its primary side to temporarily increase the voltage across the capacitor. This increases the size and cost of the transformer and capacitor, and the number of components, thereby presenting a problem of preventing the reduction in size and cost of the high intensity discharge lamp ballast apparatus.